isolation of the phenol sulfate conjugate from human urine. Several years later,
the cosubstrate 3^0 -phosphoadenosine 5^0 - phosphosulfate (PAPS) was discov-
ered (Robbins and Lipmann, 1957), and the superfamily of enzymes namely
the sulfotransferases (SULTs) has now been characterized to a great extent
(Coughtrie, 2002). The sulfonation reaction is described in detail here, along
with a summary of the SULT superfamily nomenclature and family members.
Analysis of sulfonate conjugates with examples is also discussed.
Sulfonation of chemicals involves the conjugation of the substrate with a
sulfonyl (SO 3 ) group (Fig. 3.5). The cosubstrate PAPS acts as the sulfonyl
donor and the reaction is catalyzed by a SULT enzyme. Conjugation can occur
at –C–OH, –N–OH, and –NH side chains to yield O-sulfates and N-sulfates.
PAPS is synthesized from inorganic sulfate and ATP by the enzymes
sulfurylase and adenosine 5^0 -phosphosulfate kinase in prokaryotes, and a
bifunctional enzyme PAPS synthetase (PAPSS) in higher organisms including
humans. Figure 3.6 exhibits sulfonation reactions with some common
substrates. The sulfonation reaction follows a random or ordered bi–bi
mechanism depending on the substrate and specific SULT isozyme studied, via
SN2 displacement (Gamage et al., 2005). Sulfonate transfer occurs without the
formation of intermediates. Substrate inhibition is commonly observed in this
reaction at high substrate concentrations, probably due to the formation of
dead-end complexes.
Overall reaction:p-Nitrophenol (pNP) pNP-sulfatePAPS PAPSULTOHNO 2OSO 3NO 2PAPS formationSO APS
ATP PPiSulfurylase (prokaryotes)
PAPSS (higher organisms)APS-kinase (prokaryotes)
PAPSS (higher organisms)Inorganic sulfate Adenosine 5¢-phosphosulfate 3 ¢-phosphoadenosine 5¢-phosphosulfate42 – PAPS
ATP ADPFIGURE 3.5 The overall sulfonation reaction, and formation of the cosubstrate
PAPS. PAPSS: PAPS synthetase‘‘fused’’ bifunctional enzyme, varients PAPSS1 and
PAPSS2 found in humans.
CYTOSOLIC SULFOTRANSFERASES 63